This invention relates in general to the field of heat exchange and heat exchange apparatus, especially where viscous materials must be heated. It finds particular application to coal liquefaction and liquefaction systems especially wherein a flowable mixture of fine particle size coal and a liquid hydrocarbon stream is compressed to a high pressure, heated in several heat-exchange stages to the reaction temperature, and reacted in the presence of hydrogen and a hydrogenation catalyst, with separation of the liquid and gaseous reaction products in a high temperature phase separator, and cooling of the gaseous reaction products by heat exchange with the mixture to be heated, and wherein the mixture is diluted by condensate formed during the cooling step.
Numerous methods have been proposed for obtaining liquid hydrocarbons by coal hydrogenation. In a conventional process, comminuted coal is mixed with particulate catalyst and then blended with a liquid hydrocarbon to obtain a flowable coal-oil paste-like mixture. The mixture is then compressed to a required, high process pressure usually ranging between 150 and 300 bar. During the subsequent heating to the reaction temperature of normally between 400.degree. and 500.degree. C., hydrogenating hydrogen is added to the mixture at a suitable location. Preheating of the compressed mixture is conducted in several heat-exchange stages by indirect heat exchange with gaseous reaction products. To increase the fluidity of the mixture, after at least one preheating step, it is diluted with a condensate obtained during the cooling of the hot, gaseous reaction products. For additional details of such a conventional process, attention is invited to W. Kronig, Die katalytische Druckhydrierung von Kohlen, Teeren und Mineralolen, Springer-Verlag, Berlin/Gottingen/Heidelberg, 1950.
One disadvantage of this conventional process is the fact that the preheating of the mixture is associated with a considerable pressure loss through the heat exchangers due primarily to the high viscosity of the mixture. This pressure loss is usually about 20 bar; thus increased work is necessitated to maintain the required pressure in the reactor.